In spite of the fact that the top surface of an engine cylinder block and the lower surface of an engine cylinder head may be smooth to the eye, the two surfaces never fit together closely enough to prevent leakage. Therefore, a gasket must be inserted between them to effect a seal. Various head gasket designs have been used, and these include metal head gaskets, fibrous head gaskets, sandwich type head gaskets using a metallic core and asbestos-bonded layers laminated to the core, etc. Sandwich-type gasket assemblies have sufficient resiliency to seal the cylinder head and sufficient heat resistance, especially when armored, to withstand the high temperature and pressure of the combustion chamber.
The current energy crisis has led to the development of an array of lightweight, high output, gasoline and diesel engines, both aspirated and turbocharged. One result of this shift to lighterweight engines is the increased potential for abrasive-erosion of gaskets. The phenomena of abrasive-erosion occurs in a gasket when it is placed between and in contact with the engine block and cylinder head and is then subjected to a substantial compressive load or stress and the cyclic, high frequency impulse forces and vibrations generated by the engine. These forces and vibrations tend to impart very slight relative movement between the confronting metal surfaces and the gasket. Typically, the gasket must withstand combustion pressures of 7,000 kilopascals (kPa)* in a naturally aspirated spark-ignition engine and 19,000 kilopascals or higher in a turbocharged diesel engine. Because of these high stresses, lighterweight castings and lighterweight, less rigid materials when used to form cylinder heads and engine blocks result in greater relative motion between the two bolted together components and the sealing gasket. The result of the vibratory movement is that wear and erosion of the confronting surfaces occurs and, when the gasket employs fibrous facings, the facing material begins to erode, especially at the longitudinal ends of the gasket. FNT *1 pascal=1 newton/meter.sup.2 =1.450377.times.10.sup.-4 psi
An important physical property of any gasket is good torque retention. In general, torque retention is associated with the amount of compressible material in a gasket. Compression set is defined as the residual deformation of a material after the removal of the force tending to compress it together. Specifically, if the amount of compressible material is reduced, torque retention improves. It is for this reason that thicker materials are not normally used in sealing applications requiring high torque retention. With thinner materials, however, there must be sufficient compressibility consistent with a good seal. Thus, a problem is present in maintaining good torque retention while at the same time resisting the effects of abrasive-erosion. The gasket thickness cannot simply be increased to account for wear and loss of sealing due to erosion of sealing material. Neither can an increase in the clamping forces that are used be employed to eliminate the relative movement. That simply is not practical because most applications are already at the highest possible clamping load condition.